Integrated thermal coupling for heat generating device

ABSTRACT

In order to provide a thermal coupling between a heat source and a heat sink, an integrated interleaved-fin connector is provided. A first substrate includes a first side surface and a second side surface. A plurality of heat generating devices are formed in the first side surface. A plurality of first channels are etched in the second side surface to form a plurality of first fins. A second substrate has a plurality of second channels etched therein to form a plurality of second fins and a base. The base is for thermally engaging with a heat sink. The first and second fins providing a thermally conductive path from the heat generating devices to the heat sink when interleaved with each other.

FIELD OF THE INVENTION

This invention relates generally to conductive cooling of heatgenerating devices, and more particularly to thermally coupling heatsources to heat sinks.

BACKGROUND OF THE INVENTION

It is a problem to remove heat from systems using densely packed,high-power devices. Many prior art systems use convection cooling toremove heat. To work efficiently, systems cooled by convection currentsrequire means, such as fans or pumps, to move large amounts of a coolingfluid across the heat generating devices. This makes convection coolinginappropriate for compact low-profile portable computer systems equippedwith high-speed CPU chips.

In compact systems, heat sinks are typically connected to the heatgenerating devices so that the heat can be conducted, instead ofconvected away. Frequently it is desired to construct the system so thatthe heat sink and the heat generating device can be decoupled. Thismakes servicing, repairing, and replacing of the heat generating deviceseasy.

In addition, it also desired to reduce stress at the thermal connection.The stress can be caused by differences in thermal expansion rates ofthe heat generating and removal components and by mechanical shock andvibration. Furthermore, in order to construct slim portable systems, thetotal vertical height of component assemblies must be maintained at aminimum.

In the prior art, interleaved-fin thermal connectors have been used toprovide a thermal conductive path between the heat generating device andthe heat removal mechanism. For example, U.S. Pat. No. 4,800,956,"Apparatus and Method for Removal of Heat from Packaged Element", and5,083,373 "Method for Providing a Thermal Transfer Device for theRemoval of Heat from Packaged Elements" describe thermal transferassemblies including two sets of cooling fins. The fins can beinterleaved with each other to provide a detachable thermal connection.

There are several problems with these prior art thermal connectors.First, the assembly of the elements requires several steps which do notreadily admit automation. For example, the fabrication of the prior artinterleaved devices requires a tape material to hold the fins in placeduring assembly. For example, the tape is intertwined between the fins.Next, the fins can be forced against a base plate by a jig. While heldin this position, the fins can be joined to the base plate using solder.Once the fins are fixed to the base, the tape can be removed.

Second, the prior art fins and bases are made of heat conducting metals,for example, copper or specially prepared aluminum. These metals arewell suited for fabrication and soldering of large scale components.However, it would be extremely difficult to make copper or preparedaluminum fins having a vertical height in the range of millimeters orless, and widths measured in terms of microns.

Even if rigid small fins and bases could be constructed, connecting themetallic fins and bases would be extremely difficult using the jiggedtape and soldering methods described above. Such methods are not suitedfor low-cost mass production techniques.

Third, there may be differences between thermal expansion rates of thesemiconductor devices and the fins and bases of the prior artassemblies. These differences would stress the joint where theassemblies are attached, leading to possible failures.

Therefore, it is desired to provide a detachable thermally conductivepath from the heat source directly to the heat sink. Furthermore, thepath should have a small vertical dimension. In addition, it is desiredthat the thermal path has freedom of movement in a maximum number ofdifferent directions.

SUMMARY OF THE INVENTION

In order to provide a direct thermal coupling between a heat source anda heat sink, an integrated interleaved-fin connector is provided. Theconnector comprises first and second etchable substrates. The firstsubstrate includes a first side surface and a second side surface. Aplurality of heat generating devices are formed in the first sidesurface. A plurality of first channels are etched in the second sidesurface to form a plurality of first fins. The second substrate has aplurality of second channels etched therein to form a plurality ofsecond fins and a base. The base is for thermally engaging with a heatsink. The first and second fins provide a thermally conductive path fromthe heat generating devices to the heat sink when interleaved with eachother.

In a preferred embodiment, the substrates are anisotropically etchable.For example, the substrates can be crystals, e.g., silicon basedsemiconductor substrates, polycrystals, amorphous glass, or ceramics.Furthermore, the channels are preferably wet etched at an angle which issubstantially perpendicular to surfaces of the substrates.Alternatively, the channels can be dry etched using plasma or reactiveion anisotropic etching techniques. In one aspect of the invention, thefirst and second fins are planar and parallel to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view of an arrangement including an integratedinterleaved-fin thermal connector according to a preferred embodiment ofthe invention;

FIG. 2 is a planar view of a silicon wafer used to form the substratesof thermal connector of FIG. 1;

FIG. 3 is an end view of the wafer of FIG. 2 having etched fins; and

FIG. 4 is an end view of two wafers bonded to each other.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Now with reference to the drawings, etched and integratedinterleaved-fin thermal connectors according to the preferredembodiments of the invention are described. As shown in an arrangement100 of FIG. 1, a plurality of heat generating devices 111 areconstructed on an active side surface 112 of a first substrate 110 toform a heat source.

The heat generating devices 111 can be electronic circuits, e.g.,circuits of a microprocessor, micro-mechanical devices, such as motors,or radiation generating or absorbing devices, for example, lasers. Inthe preferred embodiment of the invention, the heat generating devices111 are fabricated on an etchable substrate. For example, the substrate110 can be a crystal, e.g., a silicon based semiconductor substrate or"wafer," a polycrystal, an amorphous glass substrate, or a ceramicsubstrate.

To ensure a normal operation, heat generated by the devices 111 must beremoved. In a preferred embodiment of the invention, heat is conductedto a heat sink 130 using an integrated interleaved-fin connector.

According to the preferred embodiment, a first element of the thermalconnector is integrated into the substrate 110 of the heat source asfollows. A plurality of channels 113 are anisotropically etched into aninactive side surface 114 of the heat source 110. The channels 113 forma plurality of fins 115.

A second element of the connector comprises a second etchable substrate120. The starting material for the second substrate 120 is chosen toprovide a close match to the coefficient of thermal expansion of thefirst substrate 110. A plurality of channels 116 are etched into thesecond substrate 120 to form a plurality of fins 117 and a base 118. Thebase 116 can be thermally coupled to the heat sink 130. The thermallyconductive path is completed by interleaving the fins 115 and 117 of thefirst and second elements.

Preferably, the fins 115 and 117 are formed in the substrates 110 and120 using either wet or dry anisotropic etching techniques.

The starting material for the substrates 110 and 120 can be a crystal ofsilicon grown as an ingot. This material is readily available. Crystalsare characterized by the periodic arrangement of atoms in a regularizedlattice. The lattice of a silicon crystal, for example, can berepresented as two interpenetrating face-centered cubic lattices. Theplanes of the cubic lattices typically are described by sets of threeintegers called the Miller indices, e.g., {100}, {111}, and {110} etc.,see R.. A. Latakia, The growth of single crystals, Prentice Hall, 1970.

The crystal's growth is strongly ordered along the "faces" of thelattice. Because of this, the strength of the bonds between co-planaratoms is many orders of magnitude greater than bonds among atoms lyingin adjacent planes. This natural phenomena, exploited by the invention,leads to what are commonly known as cleavage planes.

Normally, single crystal silicon wafers are commercially available with{111} and {100} orientation of the planes of the faces, or lessfrequently with {110} orientation. Depending on the particulararrangement of the fins 115 and 117 desired, and the etching technique,various other orientations can be used, as described below.

As shown in FIG. 2, a wafer 250 is sliced from the silicon ingot to thedesired thickness. In the preferred embodiment of a low-profileconnector, the finished wafers 250, after slicing and polishing have athickness in the range of 0.5 to 1.0 millimeter for four inch standarddiameter wafers. However, the invention can also be worked with wafershaving other sizes. One or more flat edges 249 can be ground on thewafer to indicate the orientation of one of the lattice planes of thecrystal.

The heat generating devices 111 can be formed on the active side surface112 of the wafer using conventional semiconductor fabricationtechniques. Typically, electronic circuits are created by multipledepositing and etching steps. Electro-mechanical devices can bemicro-machined. Depending on the technique used, the heat generatingdevices 111 can be fabricated before or after the fins 115 are etched asdescribed below.

Now, with reference to FIG. 3, the preparation and processing of thewafer 250 is described in greater detail. First, steam oxidation can beused to form a thin silicon dioxide (SiO₂ layer 252 on the inactive sidesurface 114 of wafer 250. The SiO₂ provides a stable protective film.Next, a photo-resist layer 251 is applied on the silicon oxide layer252.

A mask, not shown, determines the desired pattern of the fins 115 ofFIG. 1 to be formed in the substrate 250. For example, the partialpattern 260 shown greatly enlarged in FIG. 2. For silicon with the {110}orientation, the pattern 260 is substantially aligned with one of the{111} cleavage planes of the lattice which perpendicularly intersectsthe surface of the wafer 250. The orientation of one such plane isgenerally indicated by the flat edge 249 ground on the otherwisecircular wafer 250.

Photo-lithography can be used to transfer the pattern of the mask to thephoto-resist layer 251. The exposed mask is further developed andprocessed to remove the mask in areas where the pattern is to be etched.

Next, the exposed wafer is immersed in a bath of hydrofluoric acid toremove the protective oxide layer in the exposed area of thephoto-resist layer 251. After the oxide layer has been selectivelyremoved to expose the pattern on the silicon, the rest of the mask layer251 can be stripped.

The channels 254 which separate the fins 255 can be formed in the wafer250 using anisotropic etching techniques. For example, in a wet etchingtechnique for wafers having a {110} orientation, the wafer 250 is simplybathed in a heated solution of potassium hydroxide (KOH). Withanisotropic etching, the etching proceeds substantiallyuni-directionally in a direction normal to the surface of the wafer. TheSiO₂ protective layer 252 prevents etching of the fins 255.

As shown in FIG. 3, the anisotropic etching yields substantiallyvertical walls 253 of the channels 254, except in the deepest portion.The channels 254 are etched into the wafer 250 in the areas where thesilicon dioxide has been removed. As an advantage of the invention,aligning the pattern 260 with the perpendicularly oriented latticeplanes yields deep and narrow channels. The rate of etching in thevertical direction is several hundred times the rate of etching in thehorizontal direction.

The unetched portions 255 of the wafer 250 form the fins 115 and 117 ofFIG. 1. The depth of the channels 254 can be controlled by the length oftime that the wafer 250 is immersed in the KOH etching solution. Thewidth of the alternating fins and channels can be in the range of, forexample, 50 to 100 microns. Once the wafer 250 has been etched, thesubstrates can be cut out of the wafer to appropriate sizes.

In the case of wafers having other orientations of cleavage planes, orin the case of substrates made of polycrystals, glass or ceramics, thefins 115 and 117 can be formed using plasma or reactive ion dry etchingtechniques. In plasma etching, a glow discharge is utilized in a partialvacuum to produce chemically reactive species, e.g., atoms, radicals, orions, from a relatively inert gas. The gas, for example, fluorine (CF₄),is selected to react with the substrate. In addition to the oxide layer252, it may be necessary to use chemical vapor deposition techniques toform a more resistant nitride protective layer on the outside of thewafer for plasma etching techniques since dry etching tends to be morecorrosive than the KOH used in the wet etching described above.

As disadvantages, dry etching requires a complex reactor, and it is moredifficult to produce deep vertical channels. However, as an advantagewith dry etching, the pattern of the mask does not necessarily need toalign with the cleavage planes of the crystal. Therefore, it is possibleto have arrangements other than parallel planar fins.

FIG. 4 shows an alternative embodiment of the integrated thermalconnector according to the invention. Here, a substrate 400 includes afirst part 410 bonded to a second part 420. The first part 410 is apolished wafer made of a silicon crystal having a {100} orientation.This type of crystal is typically used for fabricating semiconductordevices.

A second part 420 is made of a silicon wafer having a {110} orientation.This means that some of the cleavage planes of the crystal lattice areperpendicular to the side surface of the wafer. The thickness of thesecond part can be several orders of magnitude greater than thethickness of the first part 410. One side surface 421 is also polishedto a high degree of flatness.

The two parts can be joined together by a bond 430. The bond 430 can beformed by separately exposing the surfaces 411 and 421 to be bonded toan oxidizing agent in a near vacuum at a high temperature, e.g. about1000° Centigrade. This forms a silicon dioxide layer having a thicknessmeasured in atomic units. Once the surfaces 411 and 421 have beenoxidized, they can be placed in contact with each other to form a singlesilicon dioxide bond. The strength of this bond can be further increasedby annealing the assembly at elevated temperatures (>300° C). Afterannealing, the part 410 can be thinned by grinding and polishing.

The heat generating devices 111 can be formed on the part 410, and thefins can be etched on the part 420. Depending on the processingsequence, it may be necessary to protect the heat generating devices 111during fin etching.

Specific implementations of the invention have been described withrespect to an integrated heat connector of a heat generating device. Itshould be noted that other orientations of fins can also be used. Forexample, the fins can be arranged as concentric cylindrical fins toallow a relative rotation between the heat source and the heat sink.

Additional etched finned couplings can be placed between the source andsink to achieve additional degrees of freedom of movement. Thosefamiliar with the art will appreciate that it may be practiced in otherways while still remaining within the scope and spirit of the appendedclaims.

We claim:
 1. An apparatus for thermally coupling a heat source to a heatsink, comprising:a first substrate including a first side surface and asecond side surface, a plurality of heat generating devices formed inthe first side surface, and a plurality of first channels etched in thesecond side surface to form a plurality of first fins; a secondsubstrate having a plurality of second channels etched therein to form aplurality of second fins and a base for thermally engaging with a heatsink, the first and second fins providing a thermally conductive pathfrom the heat generating device to the heat sink when interleaved witheach other.
 2. The apparatus of claim 1 wherein the first and secondsubstrates are made of a silicon crystal and wherein the first channelsare anisotropically wet etched.
 3. The apparatus of claim 2 wherein thefirst and second channels are etched along planes perpendicular to thefirst and second surfaces.
 4. The apparatus of claim 1 wherein the firstand second substrates are made of a polycrystal and wherein the firstand second channels are anisotropically dry etched.
 5. The apparatus ofclaim 4 wherein the dry etching uses plasma or reactive ions.
 6. Theapparatus of claim 1 wherein the first and second fins are planar andparallel to each other.
 7. The apparatus of claim 1 wherein the firstsubstrate further comprises:a first part made of a silicon crystalhaving a {100} lattice orientation to form the plurality of heatgenerating devices; a second part made of a silicon crystal having a{110} lattice orientation to form the plurality of fins, said first andsecond part joined to each other by a bond.
 8. The apparatus as of claim7 wherein the bond is a silicon dioxide or diffusion bond.
 9. Theapparatus of claim 1 wherein the heat generating devices areelectrically powered.